Silicon limitation of biogenic silica production in the Equatorial Pacific

During the EBENE cruise (November 1996), distributions of biogenic silica c oncentration and production rates were investigated in the surface waters o f the equatorial Pacific (180 degreesW, from 8 degreesS to 8 degreesN), wit h particular emphasis on the limitation of the biogenic silica production b y ambient silicic acid concentrations. integrated over the depth of the eup hotic layer, concentrations of biogenic silica and production rates were ma ximum at the Equator (8.0 and 2.6 mmol m(-2) d(-1)) and decreased more or l ess symmetrically polewards. Contribution of diatoms to the new production was estimated indirectly, comparing biogenic silica production rates and av ailable data of new and export production in the same area. This comparison shows that new production in the equatorial area could mostly be sustained by diatoms, accounting for the major part of the exported flux of organic carbon. Kinetics experiments of silicic acid enrichment were performed. Hal f saturation constants were 1.57 muM at 3 degreesS and 2.42 mum at the Equa tor close to the ambient concentrations. The corresponding V-max values for Si uptake were 0.028 h(-1) at 3 degreesS and 0.052 h(-1) at the equator. E xperiments also show that ill situ rates were restricted to 13-78% of V-max , depending on ambient silicic acid concentrations. This work provides the first direct evidence that the rate of Si uptake by diatom populations of t he equatorial Pacific is limited by the ambient concentration of silicic ac id. However, such Si limitation might not be sufficient in itself to explai n the low diatom growth fates observed, and additional limitation is sugges ted. One hypothesis that is consistent with the results of Fe limitation st udies is that Fe and Si limitations may interact, rather than just being a mutually exclusive explanation for the HNLC character of the system. (C) 20 00 Elsevier Science Ltd. All rights reserved.